This invention relates to a variable voltage and variable frequency high frequency pulse width modulation inverter (hereinafter referred to merely as a VVVF inverter) for controlling the speed of a vehicle driving induction motor (hereinafter referred to merely as an IM).
In driving an electric vehicle by controlling the IM with the VVVF inverter, a predetermined slip frequency control operation is carried out under the following conditions:
During powering, EQU fINV=fR+fS (1)
During regeneration, EQU fINV=fR-fS (2)
where fINV is the output frequency of the VVVF inverter, i.e., the frequency of the revolving magnetic field of the IM, fR is the rotation frequency of the IM, and fS is the slip frequency. At the same time, the output voltage V of the VVVF inverter is controlled such that the ratio (V/fINV) of the output voltage V to the output frequency fINV is constant, whereby the IM output torque T is made constant. In the range where fS/fINV is small, the output torque T can be represented by the following expression: EQU T=k.multidot.V/fINV.multidot.fS (3)
Thus, if fS and V/fINV are made constant in the range where fS/fINV is small, the output torque T can be made constant according to expression (3).
FIG. 1 shows one example of the main circuit for the VVVF inverter and the IM. In FIG. 1, reference numeral 1 designates a DC source; 2, a circuit breaker; 3, a filter reactor; 4, a filter capacitor; 5, the VVVF inverter; 6, the IM; and 7, a frequency sensor for detecting the rotary frequency fR of the IM 6.
In accelerating the IM 6 in FIG. 1, as the rotary frequency fR is increased, the output frequency fINV of the VVVF inverter is increased according to the expression (1), and the inverter output voltage V is increased proportionally.
One example of a method of adjusting the output voltage V and the output frequency of the inverter is as shown in FIG. 2.
In FIG. 2, switching is generally carried out so that, when the output voltage is low and the output frequency fINV is low, the number N of high frequency pulses occurring in a half cycle of the inverter output AC waveform is decreased as the output frequency fINV of the output voltage V is increased. This is because the switching speed of an output voltage control element such as a thyristor forming the VVVF inverter is limited, and it is impossible to provide a very high modulation frequency (N.times.fINV).
FIG. 3 shows one example of a conventional 9-pulse mode modulation system in which the number of pulses N=9. More specifically, shown in FIG. 3 is a sine wave modulation system in which small harmonic components are included in the output voltage waveform. In FIG. 3, reference numeral 8 designates a triangular carrier wave; 9, a sine-wave U-phase modulation wave; 10, a U-phase modulation signal which is provided by subjecting the carrier wave 8 and the U-phase modulation wave 9 to comparison; and 11, a V-phase modulation signal which is obtained by comparing the carrier wave 8 and a V-phase modulation wave (not shown) which lags the U-phase modulation signal by 120.degree. in phase.
When the U-phase modulation signal 10 is applied as an on-off signal to the U-phase output voltage control element in the inverter 5 in FIG. 1, and the V-phase modulation signal 11 is applied to the V-phase output voltage control element, a U-V phase output voltage waveform 12 of the VVVF inverter is obtained as shown in FIG. 3.
In FIG. 3, as the height of the U-phase modulation wave 9 is increased, the slots in the output voltage waveform 12 are decreased, so that the magnitude of the VVVF inverter output voltage V can be adjusted. The inverter output frequency fINV can be adjusted by changing the frequencies of the carrier wave 8 and the U-phase modulation wave 9.
FIG. 4 is a waveform diagram showing the content factor kn of the fundamental wave which is an effective component among the respective output waveforms for the cases where the pulse members are 9, 5 and 3, respectively. More specifically, FIG. 4 shows the content factor kn of the fundamental wave in the VVVF inverter output voltage V. In FIG. 4, for the case where the pulse number is 3, the content factor kn at the maximum voltage of the output V is determined as 1. Different pulse numbers provide different content factors kn with respect to the height A of the modulation wave 9. Accordingly, if the same height A is maintained when the pulse number is switched, the output voltage V is varied, as a result of which an overvoltage is applied to the IM 6 and an overcurrent flows therein. Accordingly, whenever the number of pulses is changed, the height A is switched as indicated in FIG. 2.